High density spin-transfer torque MRAM process

ABSTRACT

A STT-MRAM integration scheme is disclosed wherein the connection between a MTJ and CMOS metal is simplified by forming an intermediate via contact (VAC) on a CMOS landing pad, a metal (VAM) pad that contacts and covers the VAC, and a MTJ on the VAM. A dual damascene process is performed to connect BIT line metal to CMOS landing pads through VAC/VAM/MTJ stacks in a device region, and to connect BIT line connection pads to CMOS connection pads through BIT connection vias outside the device region. The VAM pad is a single layer or composite made of Ta, TaN, or other conductors which serves as a diffusion barrier, has a highly smooth surface for MTJ formation, and provides excellent selectivity with refill dielectric materials during a chemical mechanical polish process. Each VAC is from 500 to 3000 Angstroms thick to minimize additional circuit resistance and minimize etch burden.

This is a continuation of U.S. patent application Ser. No. 12/290,495,filed on Oct. 31, 2008 now U.S. Pat. No. 7,884,433, which is hereinincorporated by reference in its entirety, and assigned to a commonassignee.

RELATED PATENT APPLICATION

This application is related to the following: Ser. No. 11/975,045,filing date Oct. 17, 2007; assigned to a common assignee and hereinincorporated by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to an integration scheme for a high densityspin-transfer torque (STT) MRAM device and a method of forming the samethat significantly improves final production yield and simplifiesoverall integration capability.

BACKGROUND OF THE INVENTION

Magnetoresistive Random Access Memory (MRAM), based on the integrationof silicon based Complementary Silicon-Oxide Semiconductor (CMOS) withmagnetic tunnel junction (MTJ) technology, is now a proven non-volatilememory technology with many advantages in terms of writing/read speed,power consumption, and lifetime over other commercialized memory typesincluding SRAM, DRAM, Flash, etc. However, conventional MRAM has afundamental limitation of scalability. STT-MRAM not only possesses themajor benefits of conventional MRAM but also has tremendous potentialfor scalability. Unlike conventional MRAM that requires a separate wordline in addition to a BIT line to switch the magnetization direction ofa free layer in a MTJ, STT-MRAM relies only on a current passing throughthe MTJ to rotate the free layer magnetization direction. In order forSTT-MRAM to switch a bit, however, the current density passing throughthe MTJ should be larger than a critical switching current density (Jc).Since current density is inversely proportional to device physical sizegiven a fixed amount of current, the switching efficiency increases asthe critical dimension (CD) size of the MTJ decreases. Thus, CD isnormally quite small for a STT-MRAM and is typically less than 100 nm.

A MTJ element may be based on a tunneling magneto-resistance (TMR)effect wherein a stack of layers has a configuration in which twoferromagnetic layers are separated by a thin non-magnetic dielectriclayer. In a MRAM device, the MTJ element is formed between a bottomelectrode such as a first conductive line and a top electrode which is asecond conductive line. A MTJ stack of layers that is subsequentlypatterned to form a MTJ element may be formed in a so-called bottom spinvalve configuration by sequentially depositing a seed layer, ananti-ferromagnetic (AFM) pinning layer, a ferromagnetic “pinned” layer,a thin tunnel barrier layer, a ferromagnetic “free” layer, and a cappinglayer. The AFM layer holds the magnetic moment of the pinned layer in afixed direction. In a MRAM MTJ, the free layer is preferably made ofNiFe because of its reproducible and reliable switching characteristicsas demonstrated by a low switching field (Hc) and switching fielduniformity (σHc). Alternatively, a MTJ stack of layers may have a topspin valve configuration in which a free layer is formed on a seed layerfollowed by sequentially forming a tunnel barrier layer, a pinned layer,AFM layer, and a capping layer.

The pinned layer has a magnetic moment that is fixed in the “x”direction, for example, by exchange coupling with the adjacent AFM layerthat is also magnetized in the “x” direction. The free layer has amagnetic moment that is either parallel or anti-parallel to the magneticmoment in the pinned layer. The tunnel barrier layer is thin enough thata current through it can be established by quantum mechanical tunnelingof conduction electrons. The magnetic moment of the free layer maychange in response to external magnetic fields and it is the relativeorientation of the magnetic moments between the free and pinned layersthat determines the tunneling current and therefore the resistance ofthe tunneling junction. In a read operation, when a sense current ispassed from the top electrode to the bottom electrode in a directionperpendicular to the MTJ layers otherwise known as a currentperpendicular to plane (CPP) configuration, a lower resistance isdetected when the magnetization directions of the free and pinned layersare in a parallel state (“1” memory state) and a higher resistance isnoted when they are in an anti-parallel state or “0” memory state.

During a write operation, information is written to the MRAM cell bychanging the magnetic state in the free layer from a “1” to a “0” orfrom a “0” to a “1”. In conventional MRAM, this process is accomplishedby generating external magnetic fields as a result of applying bit lineand word line currents in two crossing conductive lines, either above orbelow the MTJ element. Alternatively, in STT-MRAM, spin torquemagnetization switching is used. Spin transfer (spin torque)magnetization switching has been described by J. Sloneczewski in“Current-driven excitation of magnetic multilayers”, J. Magn. MaterialsV 159, L1-L7 (1996), and by L. Berger in “Emission of spin waves by amagnetic multiplayer traversed by a current” in Phys. Rev. Lett. B, Vol.52, p. 9353. The spin-transfer effect arises from the spin dependentelectron transport properties of ferromagnetic-spacer-ferromagneticmultilayers. When a spin-polarized current transverses a magneticmultilayer in a CPP configuration, the spin angular moment of electronsincident on a ferromagnetic layer interacts with magnetic moments of theferromagnetic layer near the interface between the ferromagnetic andnon-magnetic spacer. Through this interaction, the electrons transfer aportion of their angular momentum to the ferromagnetic layer. As aresult, spin-polarized current can switch the magnetization direction ofthe ferromagnetic layer if the current density is sufficiently high, andif the dimensions of the multilayer are small. The difference between aSTT-MRAM (also known as Spin-RAM) and a conventional MRAM is only in thewrite operation mechanism. The read mechanism is the same.

A critical current for spin transfer switching (Ic), which is defined as[(Ic⁺+I Ic⁻I)/2], for a 180 nm node sub-micron MTJ having a top-downarea of about 0.2×0.4 micron, is generally a few milliamperes. Thecritical current density (Jc), for example (Ic/A), is on the order ofseveral 10⁷ A/cm². This high current density, which is required toinduce the spin-transfer effect, could destroy a thin insulating barriermade of AlOx, MgO, or the like. In order for spin-transfer magnetizationswitching to be viable in the 90 nm technology node and beyond, thecritical current density (Jc) must be lower than 10⁶ A/cm² to be drivenby a CMOS transistor that can typically deliver 100 μA per 100 nm gatewidth.

To apply spin-transfer switching to MRAM technology, it is desirable todecrease Ic (and its Jc) by more than an order of magnitude so as toavoid an electrical breakdown of the MTJ device and to be compatiblewith the underlying CMOS transistor that is used to provide switchingcurrent and to select a memory cell.

The fabrication process of a STT-MRAM is very challenging because of thesmall MTJ size where both easy-axis and hard axis dimensions must becontrolled for optimum performance. There are two major challenges in avertical integration scheme for a STT-MRAM. The first challenge is theCD control of MTJ size and the MTJ etching process. The second challengeis fabrication of the interface between the CMOS metal layer to a MTJwithout causing any defect related issues. The first challenge wasaddressed in related MagIC patent application Ser. No. 11/975,045 whichdescribed a two mask process for forming a MTJ. However, an improvedintegration scheme for an STT-MRAM that emphasizes a better CMOS metalinterface with a MTJ cell is still needed for a production worthymanufacturing process.

A routine search of the prior art was conducted and the followingreferences were found. In U.S. Patent Application Publication2008/0089118, a method of forming a wiring to a MTJ element is shown.The MTJ is formed on a bottom electrode that is connected to a sourceregion of a transistor through a contact. The free layer has a ringshape with an insulator layer formed in the center of the ring.

U.S. Patent Application Publication 2008/0080233 discloses a method ofmaking connections to a MTJ element using a hard mask and copper vias.The MTJ contacts the top surface of a first wiring layer

SUMMARY OF THE INVENTION

One objective of the present invention is to provide an integrationscheme for a STT-MRAM device that reduces defect issues associated withone of the intermediate steps of connecting a MTJ element to a CMOSmetal pad.

A second objective of the present invention is to provide a process flowfor fabricating a STT-MRAM that improves final product yield.

According to the present invention, these objectives are achieved by afabrication sequence that sequentially forms a device array of viacontacts (VAC) on the uppermost CMOS metal layer in a substrate, a metalpad (VAM) on each active VAC in the device array, and a MTJ with anoverlying hard mask formed on each active VAM/VAC stack. Thus, the VAMserves as a metal separation layer and has a smooth, flat surface toenable smooth and flat layers in the MTJ for optimum performance. A dualdamascene process is used to connect the BIT metal layer to CMOSconnection pads. BIT lines are also formed on each MTJ/VAM/VAC stackthereby connecting a BIT line to a CMOS metal landing pad and enablingread and write processes in the STT-MRAM device.

In one aspect, the uppermost CMOS metal layer includes an array of CMOSmetal landing pads in a device region and a plurality of CMOS metalconnection pads outside the device array. A first dielectric layer isformed coplanar with the CMOS metal landing pads and connection pads andseparates adjacent metal features. Thereafter, a first etch stop layerand a second (VAC) dielectric layer are sequentially deposited on thefirst dielectric layer and uppermost CMOS metal layer. A conventionalsingle damascene process is employed to form a VAC in the VAC dielectriclayer and first etch stop layer above each CMOS metal landing pad in thedevice array. Each VAC is electrically connected through a CMOS landingpad to a transistor in a sub-structure. An intermediate separation metallayer is deposited on the VAC dielectric layer and array of VACs, and ispatterned to give a plurality of metal pads (VAM) in which each VAM padcontacts the top surface of an underlying VAC. From a top view, the areaof a VAM pad is larger than that of a VAC to ensure that the metal padcompletely covers an underlying VAC. A VAM dielectric layer is depositedto fill the openings between adjacent VAM pads and is then planarized tobecome coplanar with the VAM pads. Next a MTJ is formed on each VAM padby depositing a stack of MTJ layers on the VAM dielectric layer and onthe VAM pads and patterning the MTJ stack.

In one embodiment, the MTJ stack has a bottom spin valve configurationin which a seed layer, AFM layer, synthetic anti-ferromagnetic (SyAF)pinned layer, tunnel barrier layer, free layer, and a composite cappinglayer made of a hard mask spacer layer and an uppermost hard mask layerare sequentially formed on the substrate. All of the layers in the MTJstack may be formed by sputtering or ion beam deposition (IBD).Thereafter, the MTJ stack of layers may be annealed in an easy-axisdirection, hard-axis direction, or along both easy-axis and hard-axisdirections. The fabrication sequence comprises at least onephotolithography step and at least one etch step to form a patterned MTJelement above each VAM pad. A MTJ ILD layer is deposited on the MTJarray and is planarized to become coplanar with the plurality of MTJs.In the following steps, a second etch stop layer and a BIT dielectriclayer (BIT ILD) are sequentially deposited on the MTJ ILD and array ofMTJs. A via pattern is formed in the BIT ILD, second etch stop layer,MTJ ILD, and VAC dielectric layer to expose portions of the first etchstop layer above a CMOS connection pad. A trench is then formed in theBIT ILD layer above the via pattern and a trench is formed above eachMTJ in the device array as part of a dual damascene sequence. The etchprocess that defines the trenches also etches through the first etchstop layer in the via pattern to expose portions of the CMOS connectionpad. Then a diffusion barrier layer and BIT metal layer are deposited inthe vias and trenches to form BIT connection vias and BIT lineconnection pads above each CMOS connection pad, and a BIT line aboveeach MTJ to form an electrical connection with an underlying CMOSlanding pad through the MTJ/VAM/VAC stack

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing an intermediate step during theformation of a STT-MRAM according to the present invention in which afirst etch stop layer and a VAC dielectric layer are coated on a CMOSmetal layer.

FIG. 2 is cross-sectional view of the partially formed STT-MRAM in FIG.1 after an intermediate via contact (VAC) is formed in the VACdielectric layer and contacts each CMOS landing pad in the device array.

FIG. 3 is a cross-sectional view of the partially formed STT-MRAM inFIG. 2 after a metal separation layer is formed on the VAC dielectriclayer and contacts the top surfaces of the VACs in the device array.

FIG. 4 is a cross-sectional view of the STT-M RAM structure in FIG. 3following a process that etches the metal separation layer to form VACcover metal pads (VAM) which contact the top surface of each active VAC.

FIG. 5 is a cross-sectional view of the STT-MRAM structure in FIG. 4after a MTJ is formed above each VAM/VAC stack and within a MTJ ILDlayer.

FIG. 6 is a cross-sectional view after a second etch stop layer and aBIT ILD layer are successively formed above the MTJs and MTJ ILD layerin FIG. 5.

FIG. 7 is a cross-sectional view after a BIT connection via pattern isformed in the BIT ILD, second etch stop layer, MTJ ILD, and VACdielectric layer above a CMOS connection pad according to one embodimentof the present invention.

FIG. 8 is a cross-sectional view of the STT-MRAM structure in FIG. 7following formation of a trench above the BIT connection via pattern anda trench above each active MTJ in the device area.

FIG. 9 is a cross-sectional view showing that the via pattern andtrenches in FIG. 8 are filled with metal as a result of a dual damasceneprocess to form BIT lines above each MTJ/VAM/VAC stack and a BIT lineconnection pad and BIT connection vias above each connection pad in theCMOS metal layer.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is an integration scheme for fabricating aSTT-MRAM device that can significantly improve final production yield.The integration scheme includes a process flow that can be readilyimplemented in a manufacturing environment. Although the exemplaryembodiment describes a bottom spin valve MTJ element, the presentinvention also encompasses other STT-MRAM designs including dual spinvalve (DSF) configurations and top spin valve elements. The drawings arenot necessarily drawn to scale and the relative sizes of variouselements may differ from those in an actual device.

A method of forming a STT-MRAM structure according to one embodiment ofthe present invention will now be described. Referring to FIG. 1, apartially completed STT-MRAM structure 1 is shown that includes asubstrate comprised of a first dielectric layer 8 and an uppermost CMOSmetal layer that includes a plurality of CMOS metal landing pads 11 in adevice region 3 and a plurality of CMOS connection pads 12 in a region 2outside the device region. Only one CMOS connection pad 12 is depictedin order to simplify the drawing. There is a CMOS metal landing pad 11for each individual active MTJ element to be formed in the device areaduring subsequent process steps. In an alternative embodiment, CMOSconnection pads 12 may be in the form of metal line shapes (not shown).First dielectric layer 8 may be comprised of aluminum oxide, siliconoxide, or other insulation materials. The plane 4-4 is used only forillustration purposes to denote the boundary between regions 2 and 3. Itshould be understood that in an actual device there are a plurality ofregions 2, 3 having various shapes from a top-down view (not shown)which cannot be separated by a single plane. In FIG. 1, the drawing issimplified to focus on the critical elements of the present invention.There is preferably a diffusion barrier layer 10 along the bottom andside surfaces of CMOS metal landing pads 11 and CMOS connection pad 12to prevent metal diffusion into the adjoining first dielectric layer 8.Typically, the CMOS metal landing pads 12 are made of Cu and areelectrically connected to a transistor in a sub-structure (not shown).

A first etch stop layer 13 and a second dielectric layer 14 aresequentially formed by conventional methods on the CMOS metal layer andfirst dielectric layer 8. A plane 5-5 is shown that separates the CMOSsection of the device from the magnetic section of the device 1. Firstetch stop layer 13 has a thickness between 300 Angstroms to 1500Angstroms and may be made of silicon nitride or SiCN depending on thesize of the photoresist features printed in a subsequent lithographyprocess to form openings in the second dielectric layer 14. For example,SiCN is normally preferred for a 90 nm technology. Second dielectriclayer 14 is generally made of silicon oxide but may be comprised ofother insulation materials known by those skilled in the art.

Referring to FIG. 2, an intermediate via contact (VAC) 21 is formedabove each CMOS landing pad by a single damascene method. For example, aphotoresist layer is coated on the second dielectric layer 14 and ispatterned to form an array of openings (not shown) such that there is anopening above each CMOS landing pad 11. A reactive ion etch (RIE) stepmay be performed to transfer the openings through underlying portions ofthe second dielectric layer 14 and first etch stop layer 13. Then adiffusion barrier layer 20 may be deposited to conformally line theopenings before a VAC metal layer is deposited on the diffusion barrierlayer. In one embodiment, the VAC metal layer is comprised of Cu that isdeposited by an electroplating method. A planarization process which ispreferably a chemical mechanical polish (CMP) method is employed to makethe VACs 21 coplanar with second dielectric layer 14. Note that aplurality of dummy VACs (not shown) may be formed during the sameprocess sequence that produces active VACs 21 in order to assist the CMPprocedure in forming uniformly thick VACs across the substrate. Thedummy VACs are formed coplanar with VACs 21 but are not aligned aboveCMOS metal landing pads 11.

Since the only function of active VACs 21 is for electrically connectingMTJs formed in a subsequent step to the CMOS metal landing pads 11, theVACs should be as thin as possible, preferably 500 Angstroms to 3000Angstroms thick, to minimize the amount of additional resistance addedto the circuit and to minimize the etch burden that can arise if theaspect ratio (height/width ratio of VAC opening) is too large to permitan adequate lithography process window during the photoresist patterningstep. The advantage of inserting the VACs 21 is to separate identicalvia patterns from other patterns. In other words, one needs to deal withonly one type of via pattern during the critical MTJ layer patterningwhere a MTJ is aligned above each active VAC 21 as described in a latersection.

In FIG. 3, a metal separation layer (VAM) 31 is deposited on the seconddielectric layer 14 and plurality of VACs 21 by a physical vapordeposition (PVD) or the like. The VAM layer preferably has the followingproperties: (1) good barrier to prevent Cu diffusion from a VAC 21 intoan overlying MTJ; (2) highly uniform (smooth) to enable smooth and flatMTJ layers to be formed thereon; and (3) excellent CMP selectivitybetween the VAM layer and adjoining VAM dielectric layer 14 a (FIG. 4)so that a uniformly thick and flat surface can be obtained on thepatterned VAM pads 31 p. VAM layer 31 may be a single layer or acomposite comprised of one or more of Ta, TaN, or other conductivematerials with a thickness in the range of 100 Angstroms to 500Angstroms.

Referring to FIG. 4, the partially formed STT-MRAM 1 is depicted afterthe VAM layer is patterned to form a plurality of VAM pads 31 p in thedevice region 3. From a top view (not shown), the VAM pads 31 p may becircular, oval, rectangular, or other shapes and preferably have an areasize greater than that of the underlying VAC 21 to ensure that the VACis completely covered by the VAM pad. Thus, from a side view perspectivein FIG. 4, the width w of a VAM pad 31 p is sufficiently large to coverthe underlying VAC 21. The present invention anticipates that aplurality of dummy VAM pads may be formed in the VAM layer during thesame process steps that form VAM pads 31 p. Dummy VAM pads are used toassist in achieving more uniform VAM pads 31 p during a CMP processwhich planarizes the VAM layer with an adjoining VAM dielectric layer 14a. However, the dummy VAM pads are not electrically connected to a CMOSlanding pad 11 and are not shown to simplify the drawing.

VAM layer 31 patterning is achieved by coating and patterning aphotoresist layer on the VAM layer to form an array of openings (notshown) such that the openings uncover regions of the VAM layer that willbe removed in the following RIE step. The RIE process removesunprotected regions of VAM layer 31 and stops on second dielectric layer14. Next, a VAM dielectric layer 14 a is deposited on second dielectriclayer 14 and VAM pads 31 followed by a CMP process to coplanarize theVAM dielectric layer 14 a and VAM pads 31 p. In one aspect, the VAMdielectric layer 14 a may be made of the same material as in seconddielectric layer 14. Optionally, the insulation material in seconddielectric layer 14 may be different than that in VAM dielectric layer14 a. As indicated previously, a plurality of dummy VAM pads may beformed during the VAM layer patterning sequence in order to ensureexcellent VAM pad 31 p thickness uniformity across the substrate and toimprove flatness on each active VAM pad 31 p following the VAM CMPprocess. Dummy VAM pads are made from VAM layer 31 and are formedcoplanar with VAM pads 31 p.

Referring to FIG. 5, an MTJ stack of layers is now formed on the VAMdielectric layer 14 a and on VAM pads 31 p. Individual layers within theMTJ stack are not shown since the present invention encompasses avariety of configurations including bottom spin valve, top spin valve,and dual spin valve structures. Preferably, the MTJ stack has anuppermost capping layer comprised of a hard mask. In one embodiment, theMTJ stack has a bottom spin valve configuration in which a seed layer,AFM layer, synthetic anti-ferromagnetic (SyAF) pinned layer, tunnelbarrier layer, free layer, and a composite capping layer made of a hardmask spacer layer and an uppermost hard mask layer are sequentiallyformed on the VAM dielectric layer 14 a and VAM pads 31 p. The hard maskspacer layer may be NiCr or MnPt and the hard mask layer may be Ta, forexample.

The MTJ stack may be formed in the same process tool as the VAM 31layer. For instance, the VAM layer 31 and MTJ stack may be formed in anAnelva C-7100 thin film sputtering system or the like which typicallyincludes three physical vapor deposition (PVD) chambers each having fivetargets, an oxidation chamber, and a sputter etching chamber. At leastone of the PVD chambers is capable of co-sputtering. Usually, thesputter deposition process involves an argon sputter gas and the targetsare made of metal or alloys to be deposited on a substrate. The MTJstack of layers may be formed after a single pump down of the sputtersystem to enhance throughput.

The present invention also encompasses an annealing step after all ofthe MTJ layers have been deposited. For example, in the exemplaryembodiment, the MTJ stack of layers may be annealed in a vacuum byapplying a magnetic field of 10K Oe in magnitude along the easy axis for1 to 5 hours at a temperature of about 250° C. to 300° C. An annealprocess may also be performed along a hard-axis direction.

The MTJ stack is patterned by a process that includes at least onephotolithography step and one etching step to form a plurality of MTJelements 51. In one embodiment, a photoresist layer is coated on theuppermost layer in the MTJ stack and is patterned to form an array ofislands having the intended shape of the MTJ elements to be formed inthe following etch process. Thereafter, during a first etch step, thephotoresist layer serves as an etch mask as the pattern is transferredthrough the hard mask and stops on the hard mask spacer layer. During asecond etch step, the hard mask serves as the etch mask as the patternin the hard mask layer is transferred through the hard mask spacer andunderlying layers in the MTJ stack. It is important to select hard maskspacer and hard mask materials such as those mentioned previously thatprovide excellent etch selectivity between the hard mask layers andunderlying MTJ layers and provide excellent CMP selectivity between thehard mask and the MTJ ILD layer that is deposited once the MTJpatterning is complete. The present invention also anticipates that twolithography steps may be used to independently define the x-axis andy-axis dimension of the MTJ shape as described in related MagICapplication Ser. No. 11/975,045. In an alternative embodiment when twolithography processes are employed to define the MTJ element, a topportion of the MTJ may have a narrower width and smaller area size froma top view than a bottom portion of the MTJ.

A MTJ 51 is formed on each VAM pad 31 and is electrically connected to aCMOS landing pad 11 through a VAM pad 31 p and a VAC 21. Although theexemplary embodiment depicts the MTJ 51 as having a width v less thanthe width w of the VAM pad 31 p, the present invention also encompassesan embodiment where v≧w. Moreover, there may be a plurality of dummy MTJelements formed during the MTJ patterning sequence in order to improvethe MTJ thickness uniformity during a later CMP step that involvesplanarizing the uppermost layer in the MTJ 51. The dummy MTJ elementsare formed coplanar with MTJs 51 but are not electrically connected to aCMOS landing pad 11. The shape of MTJ 51 from a top view perspective maybe circular, oval, or other shapes used by those skilled in the art.

In the following step, a MTJ ILD layer 40 comprised of a dielectricmaterial such as aluminum oxide, silicon oxide, or a low k materialknown in the art is deposited on the MTJ 51 array and on the VAMdielectric layer 14 a by a PVD method or the like. A CMP process isperformed to make the MTJ ILD layer 40 coplanar with MTJs 51. As aresult of the fabrication sequence described herein, the plurality ofMTJ elements 51 formed on the VAM pads 31 will have a more uniform shapeand smoother surface, and improved performance than achieved by aconventional MTJ fabrication method used in constructing a standard MRAMdevice. The method of the present invention is especially advantageouswhen at least one of the hard-axis dimension and easy-axis dimension ofthe MTJs 51 is about 100 nm or less.

Referring to FIG. 6, a second etch layer 61 and a BIT ILD layer 62 aresequentially laid down on the MTJ ILD layer 40 and on MTJs 51. Thesecond etch stop layer 61 is preferably silicon nitride having athickness from 300 Angstroms to 1000 Angstroms while the BIT ILD layer62 is typically 1000 to 5000 Angstroms thick and is comprised of siliconoxide although other dielectric materials are also acceptable.

In FIG. 7, the partially formed STT-MRAM 1 is shown after a via patterncomprised of via openings is formed that uncovers portions of the firstetch stop layer 13 above CMOS connection pad 12. A conventional processis used that includes patterning a photoresist layer (not shown) on theBIT ILD layer 62 and an etch transfer through the BIT ILD layer, secondetch stop layer 61, MTJ ILD layer 40, VAM dielectric layer 14 a, andsecond dielectric layer 14. In the exemplary embodiment, two viaopenings 71 are formed above each CMOS connection pad 12. However, theremay be other embodiments where one or more than two via openings arefabricated above each CMOS connection pad 12.

Referring to FIG. 8, a second patterning and etch sequence is used todelineate a first array of trenches including trench 82 in the BIT ILDlayer 62 that extends through second etch stop layer 61. The etchprocess that opens trench 82 also removes exposed portions of the firstetch stop layer 13 and uncovers portions of CMOS connection pad 12. Inone embodiment, a trench 82 is formed above via openings 71 and a secondarray of trenches comprising a plurality of trenches 81 is formed in thedevice array such that one trench 81 is formed above each active MTJ 51and exposes the entire top surface of the underlying MTJ.

FIG. 9 shows a cross-sectional view of the completed STT-MRAM device 1after a series of process steps including deposition of a diffusionbarrier layer 90 to line the trenches 81, 82 and via openings 71, metaldeposition to fill the trenches and via openings and form via contacts93 between BIT line 92 and CMOS connection pads 12 as well as a BIT line91 above each MTJ 51, and a CMP process to coplanarize the BIT lines 91,92 with the BIT ILD layer 62.

The integration scheme described herein and the process flow employed toform the STT-MRAM structure as depicted in the embodiments of thepresent invention can significantly improve overall production yield andsimplify integration capability of the magnetic device with asub-structure including the upper most CMOS metal layer. The improvedCMOS metal interface with MTJs through the VAC and VAM connectionelements will lead to a shorter circuit with lower resistance and lowerpower consumption. Moreover, the higher degree of smoothness on the topsurfaces of the VAM pads affords higher performing MTJ cells and fewerMTJ defects during MTJ patterning and planarization processes. Forexample, the TMR ratio (dR/R) is expected to be in a significantlynarrower range for the plurality of MTJs 51 because of smoother and moreuniform thicknesses across the substrate. As a result, deviceperformance will increase and a lower rejection rate will decreaseoverall production cost compared to STT-MRAM design that do notincorporate the design improvements described herein.

While this invention has been particularly shown and described withreference to, the preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of this invention.

1. A STT-MRAM structure formed on a substrate having a CMOS metal layer including a plurality of CMOS landing pads in a device region and a plurality of CMOS connection pads outside the device region that are separated by and coplanar with a first dielectric layer, comprising: (a) a stack of dielectric layers that are formed sequentially from bottom to top on the substrate, comprising; a first etch stop layer, a second dielectric layer, a metal separation (VAM) dielectric layer, a MTJ ILD layer, a second etch stop layer, and a BIT ILD layer; (b) a plurality of intermediate via contacts (VAC) each having a first width that are formed within the first etch stop layer and second dielectric layer in the device region wherein each of said VAC is in electrical contact with an underlying CMOS landing pad, and has a top surface coplanar with said second dielectric layer; (c) a plurality of VAM pads each having a second width greater than said first width, a bottom surface contacting and completely covering a top surface of a VAC, and a top surface that is coplanar with said VAM dielectric layer; (d) a plurality of MTJ elements each having a bottom surface contacting the top surface of a VAM pad, a top surface coplanar with said MTJ ILD layer, and an easy axis dimension and a hard axis dimension; and (e) a BIT line metal layer, comprising; (1) a plurality of BIT connection vias formed coplanar with said MTJ ILD layer and within the first etch stop layer, second dielectric layer, VAM dielectric layer, and MTJ ILD layer wherein at least one BIT connection via contacts each CMOS connection pad; (2) a plurality of BIT line connection pads formed coplanar with said BIT ILD layer and within said second etch stop layer and BIT ILD layer wherein each of said BIT line connection pads contacts a top surface of at least one BIT connection via to provide an electrical connection to an underlying CMOS connection pad; and (3) a plurality of BIT lines formed coplanar with said BIT ILD layer in the device region wherein each of said BIT lines is aligned above a MTJ and is in electrical contact with said top hard mask surface to provide an electrical connection between a BIT line and an underlying CMOS landing pad through a stack of layers represented by a MTJ/VAM/VAC configuration.
 2. The STT-MRAM of claim 1 wherein each of the plurality of MTJ elements has a width that is less than the second width of a VAM pad.
 3. The STT-MRAM of claim 1 wherein the easy axis dimension and hard axis dimension in each of the plurality of MTJ elements is about 100 nm or less. 